This invention relates generally to electrothermal transfer recording media including an ink layer on one side of a substrate and an electrothermal resistive layer on the opposite side, and in particular, to an electrothermal transfer recording medium wherein the electrothermal resistive layer includes a brominated flameproofing material
Electrothermal transfer recording or printing is popular in many fields such as facsimile devices, computer terminal equipment and recorders because numerous benefits over other types of recording are obtained. The benefits include non-impact noiseless recording, minimal maintenance, low cost, reduced size, reduced weight and adaptability to multi-color printing. Most importantly, electrothermal transfer printing using an electrothermal transfer recording head is suitable for production of half tones in full-color printing and is a promising approach to production of hard copies.
FIG. 1 is a diagram illustrating the principle of operation of electrothermal transfer recording. An electrothermal transfer recording medium 101 is formed of a support layer 203 with an electrothermal resistive layer 202 on one side and an ink layer 204 on the opposite side. Resistive layer 202 has carbon black dispersed therein to impart electrical conductivity. A recording head 104 selectively applies an electric current to resistive layer 202 by a recording electrode 102 and a return electrode 103.
Electrothermal transfer recording onto a recording sheet 205 is accomplished by pressing recording electrode 102 against electrothermal resistive layer 202 to start current flow through and accumulate Joulean heat in resistive layer 202 until an elevated temperature is reached. The elevated temperature in resistive layer 202 causes substrate 203 to conduct heat to ink layer 204. As a result, ink layer 204 is heated, melted and flows onto recording sheet 205.
The most important requirements for resistive layers are generally considered to be:
(1) a resistance value between about 10.sup.2 and 5.times.10.sup.5 ohms;
(2) heat-resistance for a short period of time at a temperature of at least 300.degree. C.; and
(3) sufficient adhesion between the resistive layer and the substrate to dissipate the effect of shear friction caused by the forced contact of the resistive layer with the recording head.
To meet these requirements, a resistive layer including a resin binder and carbon black having a DBP oil absorption capacity of at leat 300 ml/100 g is disclosed and claimed in U.S. Pat. No. 4,684,563 issued on Aug. 4, 1987 to the named inventors herein.
Recently, it has become important to flameproof recording media to prevent spontaneous combustion and improve safety. Generally available flameproofing techniques require addition of a halogenated organic compound which functions as a flame retardant and an antimony oxide series compound which functions as a flame retardant assistant to a substance requiring flameproofing. However, it is difficult to disperse the necessary carbon black when these substances are added to electrothermal resistive layers. Furthermore, the halogenated organic flame retardant is not readily micronized when used in combination with resistive layers containing carbon black and therefore flameproofing properties are not imparted efficiently.
For example, when tetrabromophthalic anhydride; hexabromobenzene; or 1,2,3,4,7,8,9,10,13,13,14,14-dodecachloro-1,4,4a,5,6,6a,7,10,10a,11,12,12a -dodecahydro-1,4,7,10-dimethanodibenzo (a, e) cyclooctene (hereinafter referred to as "Dechlo"), all of which are effective flameproofing agents, are dispersed with carbon black, the dispersibility of the carbon black decreases and previously dispersed carbon black tends to flocculate and cannot be micronized. These phenomena have prevented development of flameproof electrothermal resistive layers with stable resistance values. Known resistive layers also generally have rough surfaces.
Accordingly, it is desirable to provide an electrothermal transfer recording medium having a flameproof resistive layer with a micronized flame retardant and evenly dispersed carbon black.